This invention relates to a pipette device for measuring and dispensing a selected amount of liquid. More particularly, the invention relates to an apparatus and method for withdrawing a relatively small volume of liquid from a supply liquid by means of a pipette device which has no sliding seals or plungers. Mechanical movement of the pipette device changes the internal pressure so as to withdraw and despense the selected amount of liquid.
Conventional devices for withdrawing a predetermined volume of liquid from a supply and subsequently dispensing that volume consist primarily of either common pipettes or syringe devices. The common pipette is a hollow tube having graduated markings along its length. The tip of the common pipette is inserted into the liquid supply and the internal pressure is reduced at the opposite end of the pipette, usually by the users mouth, thereby allowing atmospheric pressure to force liquid from the supply into the pipette. The user then places his finger over the end of the pipette to create a seal. By gradually permitting air to leak into the seal between the finger and the pipette end, the level of the fluid is lowered until the meniscus reaches the desired marking on the pipette. The pipette is then placed over the container where the liquid is to be dispensed and the user removes his finger thereby allowing gravity to remove the liquid from the pipette.
Syringe devices, which generally resemble the common hypodermic syringe, also comprise a hollow tube having graduated markings along its length, but also include a sliding plunger or seal within the hollow tube. With the tip of the syringe device inserted in the supply liquid, movement of the plunger away from the tip of the syringe reduces the internal pressure and liquid is thereby withdrawn from the supply into the device. By moving the plunger downward, i.e. towards the tip, the liquid is subsequently dispensed from the syringe device.
Both the common pipette and syringe devices are inherently inaccurate, especially when measuring extremely small volumes. Over a period of extended use the seals within the syringe devices wear, thereby creating leaks and concomitant inaccuracies in the measured amount of withdrawn liquid. Further, the construction of syringe devices capable of use with extremely small volumes is difficult because of the require small size of the plunger or seal.